1. Field of the Invention
This invention relates generally to a process for the continuous polymerization of an alpha-olefin in the vapor phase in a substantially horizontal, quench-cooled, stirred-bed reactor, and more particularly relates to the control of the aforesaid process to produce a solid polymer having predetermined properties.
2. Discussion of the Prior Art
Shepard et al., U.S. Pat. No. 3,957,448; Jezl et al., U.S. Pat. No. 3,965,083; Jezl et al., U.S. Pat. No. 3,970,611; Peters et al., U.S. Pat. No. 3,971,768; Stasi, U.S. Pat. No. 4,021,024; Jezl et al., U.S. Pat. No. 4,101,289; Jezl et al., U.S. Pat. No. 4,129,701; and Kreider et al., U.S. Pat. No. 4,640,963, disclose various specific embodiments of a general method performed in a substantially horizontal reactor for the vapor-phase polymerization of at least one alpha-olefin monomer in a reaction mixture comprising a first alpha-olefin monomer and, if copolymerization is occurring, a second alpha-olefin monomer. The general method disclosed comprises: conducting the polymerization under polymerization conditions of temperature and pressure in the presence of hydrogen and a catalyst system comprising a solid catalyst comprising a first meted and a cocatalyst comprising a second meted in a reactor wherein at least a portion of the heat of polymerization is removed by evaporative cooling of a readily volatilizable quench liquid, and wherein the reactor comprises a substantially horizontal reactor of substantially circular cross-section containing a centrally-located drive shaft extending longitudinally through said reactor to which are attached a plurality of adjacently located paddles, which paddles cause essentially no forward or backward movement of the particulate homopolymer or copolymer product contained in the reactor and extend transversely within and to a short distance from the internal surfaces of the reactor; driving means for the drive shaft; one or more reactor off-gas outlets spaced along the topward part of the reactor; a plurality of vapor recycle inlets spaced along the bottomward part of the reactor; one or more catalyst and cocatalyst addition inlets spaced along the reactor; a plurality of quench liquid inlets spaced along the topward part of the reactor whereby quench liquid can be introduced into the reactor; and take-off means for the particulate polymer product at one or both ends of the reactor.
In the preparation of solid polymers from alpha-olefins in a gas-phase polymerization in the aforesaid substantially horizontal, quench-cooled stirred-bed reactor, it would be highly desirable to control the polymerization reaction so as to provide a polymer product having predetermined and constant properties. In addition, a great deal of off-specification polymer product is produced during a transition period during which the polymerization conditions are changed from those employed in the manufacture of one grade of a polymer having one set of properties to those employed in the manufacture of another grade of the same polymer having a different set of properties or from those employed in the manufacture of homopolymer to those employed in the manufacture of a copolymer or vice versa.
In the operation of the aforesaid substantially horizontal, quench-cooled stirred bed polymerization reactor unit, there are a number of process changes that can lead to a situation where the process variables and product properties change with time. Several of these transient situations are operator induced while others are inherent to the process or caused by circumstances beyond the process operator's control. Among those situations controlled by the operator are: shut-down, start-up, production rate changes, and product grade changes. Grade changes tend to be very complex and time consuming. The main reason for the unusual complexity is that each grade change requires by definition that the plant's steady-state operating conditions be substantially disturbed. Furthermore, the problem is aggravated by the fact that during the transition several operating variables can change simultaneously. For example, some variables, like reactor temperature, pressure and catalyst feed, control the reaction rate, whereas others, such as hydrogen and/or propylene partial pressures, control the transition's speed and the ultimate polymer properties. Grade changes are always coupled with cost penalties due to the generation of off-specification material during the product transition.
Thus, in order to quantify the cost penalties associated with product transitions, it would be highly desirable to develop a mathematical model of the process. A critical utilization of this process model is in the form of an advanced control method that can be utilized for on-line minimization of grade transition times and to maintain safe operation from one product transition to another. It would also be desirable to reduce this transition period and the amount of off-specification polymer produced during the transition period. Thus far, no method has been disclosed for controlling the continuous vapor-phase polymerization in the aforesaid substantially horizontal, quench-cooled, stirred-bed reactor.
The desired control of the polymerization process is extremely difficult to attain because of the holdup time of polymerization reactors and the time involved in obtaining polymer samples and measuring the properties of those samples. Because of this time period, the polymerization conditions employed in the reactor at the time at which a property of a polymer sample withdrawn from the reactor is measured are not necessarily the same as the polymerization conditions employed in the reactor at the time at which such polymer sample was produced in the reactor and/or withdrawn from the reactor. This is especially the case when the attempted control of the polymerization process is based on the measurement of the melt How rate--or in other words, the melt index--of the polymer product as determined according to the ASTM Test D-1238-62T. Although the melt flow rate or the melt index is a satisfactory control property for most solid polymers prepared from alpha-olefins, the time consumed in obtaining a polymer sample for measurement and in measuring the melt index of the sample, combines with the aforesaid holdup time of the reactor to seriously hamper accurate control of the polymerization process.
Consequently, it is highly desirable to develop an advanced on-line control method for the continuous gas-phase polymerization of alpha-olefins in the aforesaid substantially horizontal, quench-cooled, stirred-bed reactor, which is based on measurements in real time of polymerization conditions in the reactor at the same time the control is being implemented. Such control methods have been disclosed for use in liquid phase polymerization reactor systems. For example, Smith et al., U.S. Pat. No. 3,356,667 discloses a method and apparatus for controlling reaction conditions of an alpha-olefin polymerization in the liquid phase in order to produce a solid polymer having specific properties. In addition to the basic reactor system, the apparatus disclosed as being useful in the practice of the disclosed method includes: means for feeding catalyst and reactant materials to the reactor system, means for withdrawing an effluent product stream from the reactor system, means for removing reaction heat from the reactor system, computing means for establishing an output control signal representative of the instantaneous melt index of the polymer being produced in the reaction mixture, a second output signal representative of the averaged melt index of the polymer in the effluent product stream removed from the reactor system, a third output signal representative of the average percent hydrogen in the reaction mixture within the reactor, and means for applying the output signal representative of the instantaneous polymer melt index to control the rate of hydrogen addition to the reactor system so as to yield a polymer product having a predetermined melt index based on a predetermined concentration of hydrogen in the reaction zone. Also disclosed are computing means for establishing an output signal representative of polymer production rate and associated means for controlling the rate of addition of catalyst to the reactor in response to the computed production rate.
Smith et al., U.S. Pat. No. 3,356,667 also discloses that both the instantaneous melt index value of the polymer in the reaction zone and the melt index value of the polymer in the effluent from the reactor can be determined by automatically computing these melt index values from the input data of various process variables of the reaction system. The computer inputs include (1) concentration of hydrogen in the monomer feed, (2) polymer concentration (percent solids) in the reactor system, (3) temperature of the reaction mixture, (4) rate of flow of the monomer to the reactor, and (5) a time factor to compensate for delay. The production rate is also disclosed as a useful input signal. Thus, regulation of the hydrogen feed rate is accomplished in response to a hydrogen analysis in the feed corrected to indicate the hydrogen concentration in the liquid phase in the reactor. In addition, output signals representative of the melt index of the polymer in the reactor effluent and the concentration of hydrogen in the reaction liquid are obtained.
The control system disclosed in the aforesaid U.S. Pat. No. 3,356,667 employs as a basic unit thereof a computer which is adapted to receive input signals representative of the flow rate of propylene fed to the reactor system, the hydrogen concentration in the propylene feed, the temperature of the reaction liquid in the reactor, and the percent solids (percent polymer) in the reaction mixture in the reactor. The signal representative of the flow rate of propylene, the signal representative of the hydrogen concentration, the signal representative of the temperature of the reaction contents in the reactor, the signal representative of the polymer concentration (percent solids), and the signal representative of production rate are transmitted to the computer. The computer accepts the input signals from the primary measurement devices and produces three principal output signals that are linearly proportional to the following process variables: (1) an output signal representing the concentration of hydrogen in the reaction liquid phase; (2) an output signal representing the instantaneous melt index of the polymer presently being produced in the reactor reaction mixture and (3) an output signal representing the integrated melt index of the polymer in the effluent stream removed from the reactor. The computer automatically combines the input signal to produce control output signals that are proportional to the instantaneous melt index, the concentration of hydrogen in the reaction liquid phase, and the melt index of the polymer in the reaction effluent and that are in response to the input signals.
A first control output signal proportional to the melt index of the polymer being produced at any instant within the reactor (instantaneous melt index) is transmitted to a melt index-recording-controller. The computed instantaneous melt index is compared by the controller with a predetermined desired polymer melt index value (set point), and a signal representative of this comparison is transmitted to manipulate the set point of an analyzer-recorder-controller. By so operating, in the method disclosed in the aforesaid U.S. Pat. No. 3,356,667, the concentration of hydrogen in the reactor is maintained at a value capable of producing a polymer product having a predetermined melt index.
Furthermore, Smith, U.S. Pat. No. 3,614,682 discloses a method for the digital computer control of a polymerization process that is performed in a continuously operating series of stirred reactors wherein each reactor continuously receives discharge of the preceding reactor in the series and continuously discharges into the succeeding reactor in the series. It is disclosed that at stated intervals, a computer begins a cycle, the first portion of which is a simulation routine whereby changes, since the last simulation, in important variables which take place at various points in the successive reactors in the train of reactors and which cannot be directly measured are followed by periodically numerically integrating by a digital computer for each reactor and for each of the variables, the equation ##EQU1## wherein, in the terms employed in U.S. Pat. No. 3,614,682, X is a process variable like concentration, conversion, etc., i is the first subscript of X and signifies that this is the ith of i variables, n is the second subscript of X and signifies that this is the value of this variable in the nth one of the reactors in the train of reactors, F is the total volumetric flow rate, V is the volume of the nth reactor, t is the time under the reaction conditions, and .delta.X.sub.i,n /.delta.t is the overall rate of generation or degeneration of X under the conditions in the nth reactor. The resulting calculated values of these variables, together with directly measured values of other variables, are then manipulated by the computer and used to adjust the rate of feed of reagents and other conditions of polymerization in the train of reactors.